Magnetic tape having a leader tape

ABSTRACT

A magnetic tape having a leader tape excellent in cleaning effect as well as antistatic effect and free from head clogging is provided comprising a coating layer provided on one or both sides of a support, at at least one end of the magnetic tape, in which the coating layer contains, as predominant components, an electrically conductive powder and a binder and the leader tape has a percent photo-transmission of at least 20%.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a magnetic tape having a leader tape at theend thereof and more particularly, it is concerned with a leader tapehaving a specified percent photo-transmission.

2. Description of the Prior Art

In magnetic recording instruments such as audio cassette recorders orvideo tape recorders, leader tapes are used in combination with the endsof magnetic tpes so as to clean dirt or dust off the recording andreproducing heads in use and the guide poles or rollers in the runningpaths and to detect the tape ends.

In the leader tapes of the prior art, it is known to provide a cleaninglayer on a support by coating an inorganic pigment such as alumina (Al₂O₃), silicon carbide (SiC), chromium oxide (Cr₂ O₃), titanium oxide(TiO₂), red oxide (α-Fe₂ O₃) or silicon oxide (SiO₂) with a binder(Japanese Patent Publication No. 7631/1980). This leader tape has acleaning effect, but meets often with the problems that due to itsstatic charging property, discharge noise tends to occur and dirt ordust is adsorbed to cause drop out. Thus, another approach consists inadding antistatic agents to the cleaning layer or coating antistaticagents onto the cleaning layer. In the case of adding an antistaticagent to the cleaning layer, however, it must be used in a large amount,so the strength of the cleaning layer is lowered and the cleaning layertends to be stripped off. When adding black conductive grains such ascarbon black and metallic fine powders, the cleaning layer has littlephoto-transmission and the leader tape cannot play a role to detect thetape end. In the case of coating an antistatic agent onto the cleaninglayer, moreover, there arise the problems that head clogging tends tooccur due to adhesion of the antistatic agent and the antistatic effectis lowered when using for a long period of time.

Furthermore, as one of the prior art, there has been proposed a methodcomprising using a mixture of inorganic and organic powders (JapanesePatent Application OPI (Kokai) No. 12411/1982). However, this method hasalso the drawback that resulting cleaning leader tape causes increase ofdrop out due to its electrification property and when it is subjected toan antistatic treatment, head clogging tends to occur.

That is, according to the prior art, it is impossible to solvesufficiently the problems in magnetic tapes having cleaning leadertapes, i.e. increase of drop out due to static charge, lowering of thestrength of the cleaning tape by the antistatic treatment, decrease ofthe photo-transmission, head clogging and lowering of the antistaticeffect after use for a long period of time.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a magnetic tapehaving a leader tape with an excellent cleaning effect.

It is another object of the present invention to provide a magnetic tapehaving a leader tape having a stable and excellent antistatic effect fora long period of time or having little electrification property andbeing free from increase of drop out.

It is further object of the present invention to provide a magnetic tapehaving a leader tape without head clogging.

It is a still further object of the present invention to provide amagnetic tape having a leader tape whose cleaning effect is notdeteriorated even after storage at a high temperature.

It is a still further object of the present invention to provide aleader tape for a magnetic tape, whereby the above describeddisadvantages of the prior art can be overcome.

These objects can be attained by a magnetic tape having a leader tapecomprising a coating layer provided on one or both sides of a support,at at least one end of the tape, characterized in that the coating layercontains, as predominant components, an electrically conductive powderand a binder and the leader tape has a percent photo-transmission of atleast 20%.

DETAILED DESCRIPTION OF THE INVENTION

The inventors have made various efforts to solve the above describedproblems and consequently, have reached the magnetic tape having aleader tape according to the present invention.

Accordingly, the present invention provides a magnetic tape having, atat least one end thereof, a leader tape comprising a coating layerprovided on one or both sides of a support, characterized in that thecoating layer contains predominantly an electrically conductive powderand a binder and the leader tape has a percent photo-transmission of 20%or more. Furthermore, the present invention has another feature that thecoating layer, i.e. cleaning layer further contains a white inorganicpowder and/or white resion powder in addition to the conductive powderand binder.

The electrically conductive powder used in the present invention ispreferably a powder having generally a mean grain size of 1 to 0.001 μm,preferably 0.7 to 0.005 μm, a volume resistance of 10° to 10⁸ Ω-cm andin a granular, global, ellipsoidal, cubic, needle-like or plate-likeform. Examples of the electroconductive powder are (1) SnO₂, In₂ O₃,ZnO, BaTiO₃ and SrTiO₃ each having conductivity and grey, blue, violetor yellow color; (2) the above described electroconductive powders (1)doped with elements with different valences to raise the conductivity,such as SnO₂ doped with 2-20 atom % of Sb⁵⁺, In₂ O₃ doped with 2-20 atom% of Sn⁴⁺, BaTiO₃, SrTiO₃ and (Ba, Sr)TiO₃, doped with 0.1-1 atom % ofLa³⁺, Y³⁺, Ce³⁺ Nd³⁺, Sb³⁺ and Bi³⁺ ; (3) electroconductive powders,such as ZnO, BaTiO₃ and SrTiO₃, into which oxygen defects are introducedby a heat treatment in a non-oxygen atmosphere, such as ZnO_(1-x')BaTiO_(3-x) and SrTiO_(3-x) wherein x=0.01-0.3, preferably 0.02-0.2, asdisclosed in Japanese Patent Application OPI (Kokai) No. 156603/1981;and (4) ordinary white pigments or extenders, such as Al₂ O₃, TiO₂,SiO₂, Sb₂ O₃, BaSO₄ and ZnO, in a fine powder having a mean grain sizeof 0.05 to 2 μm, preferably 0.01 to 1 μm, which are coated with theabove described electroconductive powders (1) to (3) to form anelectroconductive layer, preferably coated with an electroconductivelayer consisting of SnO₂ or In₂ O₃ doped with an element of differentvalence, as disclosed in Japanese Patent Application OPI (Kokai) No.11825/1982.

The above described electrically conductive powder used in the presentinvention is characterized by a higher conductivity as well as a higherpercent photo-transmission when it is dispersed in a binder and coated,as compared with the commonly used carbon, metallic fine powders andblack iron oxide (Fe₃ O₄). This characteristic is particularlyremarkable when using SnO₂ or In₂ O₃, optionally doped with differentelements.

The electrically conductive powder in the coating layer of the presentinvention may further contain a white inorganic powder and/or a whiteresin powder.

In one embodiment of the present invention, wherein the coating layercontains predominantly an electrically conductive powder, whiteinorganic powder and binder, the durability of the cleaning effect canfurther be increased in addition to the effects in the embodiment usingan electrically conductive powder and binder. In the case of a coatinglayer, i.e. cleaning layer of the prior art, using predominantly a whitepigment, drop out is increased due to the static charging property andwhen subjected to an antistatic treatment, head clogging or othertroubles take place. When the coating layer of the present inventioncontains predominantly an electrically conductive powder free fromelectrification property and a white inorganic powder, not only theelectrification property is lowered to such an extent that an antistatictreatment is not necessary as in the first embodiment of the presentinvention, but also the durability of the cleaning effect issurprisingly increased more than in the case of only an electricallyconductive powder.

Examples of the white inorganic powder used in the present invention arealumina (α-Al₂ O₃), titanium oxide (TiO₂), zinc sulfide (ZnS), lithopone(ZnS-BaSO₄), barium sulfate (BsSO₄), zinc oxide (ZnO), aluminum silicate(Al₄ (SiO₄)₃.5H₂ O), silicon oxide (SiO₂) and the like. Generally, thewhite inorganic powder has a grain size of 0.01 to 5 μm, preferably 0.1to 3 μm. These white inorganic powders can preferably be used,individually or in combination, in a proportion of 0 to 950 partspreferably 10 to 500 parts by weight to 100 parts by weight of theelectrically conductive powder.

In another embodiment of the present invention, wherein the cleaninglayer contains predominantly an electrically conductive powder, whiteresin powder and binder, there can be obtained an additional merit thata head is not scratched in addition to the effects of the firstembodiment of the present invention. In the case of a cleaning layer ofthe prior art, containing predominantly a white pigment and resinpowder, drop out is increased due to the static charging property andwhen subjected to an antistatic treatment, head clogging takes place,but there is obtained a merit that a head is not scratched. According tothis embodiment of the present invention, the cleaning layer containspredominantly a white electrically conductive powder free fromelectrification property, white resin powder and binder, thus resultingin advantages that drop out is not increased, head clogging does nottake place because of not needing an antistatic treatment and a head isnot scratched because of containing the white resin powder.

Examples of the white resin powder used in the present invention areacetylcellulose, polyethylene, polypropylene, polyester resins,polyamide resins, polyacrylonitrile resins, polyacetal resins, phenolresins, urea resins, triazine resins, melamine resins, epoxy resins andthe like. In particular, polyethylene, urea resins and triazine resinsare preferably used.

Generally, these resin powders have a grain size of 0.1 to 10 μm,preferably 0.3 to 3 μm. If the grain size is less than 0.1 μm, thecleaning effect is little, while if more than 10 μm, the tape tends tobreak in a high incidence when the leader tape is cramped by a hub.These white resin powders can preferably be used, individually or incombination, in a proportion of 0 to 450 preferably 10 to 450 parts byweight of 100 parts by weight of the electrically conductive powder.

In a further embodiment of the present invention, the coating layer onthe leader tape may contain predominantly an electrically conductivepowder, white inorganic powder, white resin powder and binder. In thiscase, there can be obtained a magnetic tape having a leader tape withexcellent properties that the electrification property is less, drop outis not increased, head clogging does not occur, the cleaning effect isdurable even after storage at 110° C. and any head is free fromscratching.

Preferably, the white resin powder and white inorganic powder arejointly used in a total quantity of 0 to 450, preferably 10 to 450 partsby weight per 100 parts by weight of the electrically conductive powder.

As the binder of the present invention, there are used known resins suchas vinyl chloride/vinyl acetate copolymers, vinylidene chloride typecopolymers, nitrocellulose, polyester resins, polyurethane resins,polyisocyanates and the like. These binders can be used, individually orin combination, in a proportion of 10 to 600 parts by weight, preferably15 to 400 parts by weight to 100 parts by weight of the powder. Thepercent photo-transmission can be varied by varying the proportion ofthe powder and binder.

The powder and binder are mixed and dispersed in an organic solvent. Asthe organic solvent, there can be used any of organic materials whichare not reactive with the powders and are capable of dissolving thebinders and which have a boiling point of 60° to 200° C., for example,ketones such as methyl ethyl ketone, methyl isobutyl ketone,cyclohexanone and the like and acetic acid esters such as ethyl acetate,butyl acetate and the like. In addition, toluene or xylene can be usedas a diluent.

The thus obtained mixed composition is coated onto, partially or wholly,one or both sides of a support with a coating thickness of 0.05 to 20μm, preferably 0.1 to 10 μm.

The support used in the present invention is a non-magnetic flexiblesupport having a haze of preferbly 80% or less, more preferably 10 to80%. If the haze is less than 10%, the leader tape meets with a markedchange of the appearance (clarification) and a marked decrease of thecleaning effect, while if more than 80%, the percent phototransmissionof the leader tape is decreased so that it is difficult to detect theend of the magnetic tape by means of a photoelectric senser, and thestrength of the leader tape is lacking.

As the support, there are preferably used films of polyethyleneterephthalate, polypropylene, polycarbonates, polyethylene naphthalate,polyamides and polyimides. The haze can be controlled by adding a whiteinorganic powder to the support or precipitating a catalyst for thepolymerization as internal grains, as occasion demands.

Examples of the support are disclosed in Japanese Patent Publication No.23888/1968, Japanese Patent Application OPI (Kokai) Nos. 122178/1976 and116066/1976 (incorporated by reference).

The smaller is the heat shrinkage of the support, the better. Afterstorage at 110° C. for 4 hours, for example, the heat shrinkage is 3% orless, preferably 1% or less. Generally, the support has a thickness of 3to 50 μm, preferably 6 to 25 82 m.

Furthermore, dispersing agents can be used for the purpose of improvingthe dispersibility of the powders in the binders. Optionally, lubricantscan jointly be used. Any of the commonly used dispersing agents andlubricants are available, as disclosed in Japanese Patent ApplicationOPI (Kokai) No. 108804/1977.

On the other hand, the leader tape of the present invention should becontrolled so as to have a percent photo-transmission of 20% or more,since if the percent photo-transmission is less than 20%, it isdifficult to detect the end of the magnetic tape by means of aphotoelectric senser.

If necessary, the leader tape of the present invention can be printedwith letters or marks, as exemplified in Utility Model Application OPI(Kokai) Nos. 2209/1976 and 72924/1978.

The following examples are given in order to illustrate the presentinvention without limiting the same. It will be obvious to one skilledin the art that various changes and modifications can be made in thecomponents, ratios, operational order and the like without departingfrom the spirit of the present invention. All parts in Examples are tobe taken as those by weight.

Electrically conductive powders used in Examples and ComparativeExamples are as follows:

a: Sb-doped SnO₂ powder (mean grain size 0.4 μm, Sb content 6 atom %)

b: TiO₂ powder coated with Sb-doped SnO₂ (Sb 0.5 atom %, Sn 7 atom %,specific surface area 6 m² /g, mean grain size 0.4 μm)

c: α-Al₂ O₃ powder coated with Sb-doped SnO₂ (Sb 0.5 atom %, Sn 7 atom%, specific surface area 6 m² /g, mean grain size 0.4 μm)

d: Sb-doped SnO₂ powder (Sb 10 atom %, mean grain size 0.2 μm)

e: Sn-doped In₂ O₃ powder (Sn 10 atom %, mean grain size 0.2 μm)

EXAMPLE 1-1

Using the above described Electrically Conductive Powders a-e, coatingcompositions for leader tapes were prepared according to the followingrecipe:

    ______________________________________                                        Electrically Conductive Powder                                                                        100    parts                                          Nitrocellulose (RS 1/2 second)                                                                        25     parts                                          Polyester Polyurethane  10     parts                                          Polyisocyanate (commercial name:                                                                      10     parts                                          Desmodur L, solid content of 75%                                              solution)                                                                     Methyl Ethyl Ketone     500    parts                                          ______________________________________                                    

The above described composition was well blended to form a coatingcomposition for a leader tape and coated onto a support of polyethyleneterephthalate with a thickness of 20 μm to give a coating thickness of 8μm on dry basis, followed by drying. This tape was slit in a width of3.8 mm to obtain a leader tape for a compact cassette.

This leader tape was cramped with a hub to examine the cramp force andthe state of breakage of the tape at that time, and loaded in a 1 mcassette half, which was then subjected to running in a cassetterecorder having a magnetic head previously made dirty, to examine thenumber of times required for removing the dirt. Thus, the less thenumber of times required for removing the dirt, the larger the cleaningeffect. At the same time, head clogging was examined after running10,000 times.

On the other hand, the cleaning effect was examined after running. Theleader tape before and after running was rubbed with a cotton bloth toexamine the electrification property by a distance at which a cigaretteash was attracted when the rubbed leader tape was put over the cigaretteash. The longer the attraction distance, the stronger the staticcharging property.

The thus obtained results are shown in Table 1 by Sample Nos. 1-5.

COMPARATIVE EXAMPLE 1-1

The procedure of Example 1-1 was repeated except using the followingwhite powders instead of Electrically Conductive Powders to obtainleader tapes:

f: TiO₂ (mean grain size 0.5 μm)

g: α-Al₂ O₃ (mean grain size 0.5 μm)

h: Triazine Resin Powder (mean grain size 2.5 μm; true specific gravity1.4)

The propeties of the resulting leader tapes are shown in Table 1 asSample Nos. 6-8.

COMPARATIVE EXAMPLE 1-2

Antistatic agent of quaternary ammonium salt type (commercial name:EFUKOORU 72 manufactured by Matsumoto Yushi KK) was coated onto thesurfaces of the leader tapes of Comparative Example 1-1 in a proportionof 200 mg/m².

                                      TABLE 1                                     __________________________________________________________________________                                            After Running 10,000 Times                               Breakage        Electri-               Electri-                         Percent                                                                             Ratio at   Cleaning                                                                           fication          Cleaning                                                                           fication                         Phototrans-                                                                         Cramping                                                                            Cramp                                                                              Effect                                                                             Property                                                                           Head Missing of                                                                            Effect                                                                             Property            Sample No.                                                                            Powder                                                                             mission (%)                                                                         (%)   Force (g)                                                                          (times)                                                                            (cm) Clogging                                                                           Coating Layer                                                                         (times)                                                                            (cm)                __________________________________________________________________________    Example 1-1                                                                   1       a    77    0     1800 or                                                                            1    0.0               2    0                                            more                                                 2       b    79    0     1800 or                                                                            1    0    "    "       2    0                                            more                                                 3       c    80    0     1800 or                                                                            1    0    "    "       2    0                                            more                                                 4       d    81    0     1800 or                                                                            1    0    "    "       2    0                                            more                                                 5       e    81    0     1800 or                                                                            1    0    "    "       2    0                                            more                                                 Comparison 1-1                                                                6       f    78    0     1800 or                                                                            1    10   "    "       2    10                                           more                                                 7       g    79    0     1800 or                                                                            1    9    "    "       2    10                                           more                                                 8       h    83    0     1800 or                                                                            2    12   "    "       4    12                                           more                                                 Comparison 1-2                                                                9       f    78    0     1800 or                                                                            1    0.5  somewhat                                                                           "       2    5.0                                          more                                                 10      g    79    0     1800 or                                                                            1    0.5  somewhat                                                                           "       2    6.0                                          more                                                 11      h    83    0     1800 or                                                                            2    0.5  somewhat                                                                           "       4    5.5                                          more                                                 __________________________________________________________________________

EXAMPLE 1-2

Using Electrically Conductive Powder a, the procedure of Example 1-1 wasrepeated except changing the total weight parts of Nitrocellulose,Polyester Polyurethane and Polyisocyanate and the weight part of thesolvent without changing the relative proportion of Nitrocellulose,Polyester Polyurethane and Polyisocyanate (50:30:20), thus obtainingleader tapes. The total weight parts and the properties of the resultingleader tapes are shown in Table 2 as Sample Nos. 12-20.

                                      TABLE 2                                     __________________________________________________________________________                                            After Running 10,000 Times                   Total       Breakage        Electri-               Electri-                   Amount of                                                                           Percent                                                                             Ratio      Cleaning                                                                           fication          Cleaning                                                                           fication            Sample Binder                                                                              Phototrans-                                                                         Cramping                                                                            Cramp                                                                              Effect                                                                             Property                                                                           Head Missing of                                                                            Effect                                                                             Property            No.    (parts)                                                                             mission (%)                                                                         (%)   Force (g)                                                                          (times)                                                                            (cm) Clogging                                                                           Coating Layer                                                                         (times)                                                                            (cm)                __________________________________________________________________________    Example 1-2                                                                   12      25   76    0.0   1600 1    0    no   no      1    0                   13      10   70    0     1500 1    0    "    yes     1    0                   14      15   75    0     1550 1    0    "    somewhat                                                                              1    0                   15      50   77    0     1800 or                                                                            "    0    "    no      2    0                                            more                                                 16     100   78    0     1800 or                                                                            "    0    "    "       "    0                                            more                                                 17     200   78    0     1800 or                                                                            "    0    "    "       "    0                                            more                                                 18     400   79    0     1800 or                                                                            "    0    "    "       "    0                                            more                                                 19     600   80    0     1800 or                                                                            2    0.5  "    "       3    0.5                                          more                                                 20     800   82    0     1800 or                                                                            3    0.5  "    "       5    0.5                                          more                                                 __________________________________________________________________________

It will clearly be understood from Table 1 that the leader tapes of thepresent invention (Sample Nos. 1 to 5) have less electrificationproperty and more excellent cleaning effect than that of the prior art(Sample Nos. 6 to 8). The leader tapes subjected to the antistatictreatment of the prior art (Sample Nos. 9 to 11) meet with head cloggingand deterioration of the antistatic effect. Thus, the leader tapes ofthe present invention are more excellent in cleaning effect as well asantistatic effect and do not or hardly meet with head clogging anddeterioration of the cleaning effect and antistatic effect.

Furthermore, it is apparent from Table 2 that the amount of the binderin the coating layer of the leader tape is preferably about 15 to 400parts per 100 parts of Electrically Conductive Powder, according to thepresent invention.

EXAMPLE 1-3

The leader tapes of Example 1-1 were slit in a width of 1/2 inch toobtain leader tapes for video tapes. The resulting leader tapes werecombined with hubs in the order of 17 cm of Leader Tape, 247 m of VideoTape and 17 cm of Leader Tape. The resulting VHS video cassette tape wassubjected to running in a VHS video deck 200 times for assessment of theperformances.

The results are shown as Sample Nos. 21 to 25 in Table 3.

COMPARATIVE EXAMPLE 1-3

The leader tapes of Comparative Example 1-1 were slit in a width of 1/2inch to obtain leader tapes for video tapes and then subjected toassessment in an analogous manner to Example 1-3. The results are shownas Sample Nos. 26 to 28 in Table 3.

COMPARATIVE EXAMPLE 1-4

The leader tapes of Comparative Example 1-2 were slit in a width of 1/2inch to obtain leader tapes for video tapes and then subjected toassessment in an analogous manner to Example 1-3. The results are shownas Sample Nos. 29-31 in Table 3.

                  TABLE 3                                                         ______________________________________                                                    White       Head     Increase of                                  Sample No.  Powder      Clogging Drop out                                     ______________________________________                                        Example 1-3                                                                   21          a           no       no                                           22          b           "        "                                            23          c           "        "                                            24          d           "        "                                            25          e           "        "                                            Comparison 1-3                                                                26          f           "        yes                                          27          g           "        "                                            28          h           "        "                                            Comparison 1-4                                                                29          f           yes      somewhat                                     30          g           "        "                                            31          h           "        "                                            ______________________________________                                    

Examples of the white inorganic powder used in the following Examplesand Comparative Examples are as follows:

i: α-Al₂ O₃ (mean grain size 0.3 μm)

j: TiO₂ (mean grain size 0.3 μm)

k: SiO₂ (mean grain size 0.1 μm)

l: BaSO₄ (mean grain size 0.2 μm)

EXAMPLE 2-1

    ______________________________________                                        White Electrically Conductive Powder                                                                    50      parts                                       (as shown in Table 4)                                                         White Inorganic Powder (as shown in Table 4)                                                            50      parts                                       Vinyl Chloride-Vinyl Acetate Copolymer                                                                  35      parts                                       (composition 87:13; degree of polymerization                                  430)                                                                          Polyester Polyurethane (molecular weight                                                                10      parts                                       150,000)                                                                      Polyisocyanate (commercial name: Desmodur L;                                                            5       parts                                       solid content of 75% solution)                                                Methyl Ethyl Ketone       2700    parts                                       ______________________________________                                    

The above described components were well mixed to prepare coatingcompositions for leader tapes and processed in an analogous manner toExample 1-1, to obtain leader tapes for compact cassettes.

The resulting leader tapes were subjected to assessment in an analogousmanner to Example 1-1, thus obtaining results as shown in Table 4 asSample Nos. 32 to 39.

It is apparent from Table 4 that the leader tapes using theelectroconductive powders and white inorganic powders according to thepresent invention are more excellent in cleaning effect as well asantistatic effect and these effects are not changed even after running10,000 times.

                                      TABLE 4                                     __________________________________________________________________________                                            After Running 10,000 Times                   White                       Electri-               Electri-                   Electrically                                                                        White Percent    Cleaning                                                                           fication          Cleaning                                                                           fication            Sample Conductive                                                                          Inorganic                                                                           Phototrans-                                                                         Cramp                                                                              Effect                                                                             Property                                                                           Head Missing of                                                                            Effect                                                                             Property            No.    Powder                                                                              Powder                                                                              mission (%)                                                                         Force (g)                                                                          (times)                                                                            (cm) Clogging                                                                           Coating Layer                                                                         (times)                                                                            (cm)                __________________________________________________________________________    Example 2-1                                                                   32     a     i     76    1800 or                                                                            1    0    no   no      1    0                                            more                                                 33     b     i     73    1800 or                                                                            1    0    "    "       1    0                                            more                                                 34     c     i     74    1800 or                                                                            1    0    "    "       1    0                                            more                                                 35     d     i     78    1800 or                                                                            1    0    "    "       1    0                                            more                                                 36     e     i     78    1800 or                                                                            1    0    "    "       1    0                                            more                                                 37     a     j     77    1800 or                                                                            1    0    "    "       1    0                                            more                                                 38     a     k     80    1800 or                                                                            1    0    "    "       1    0                                            more                                                 39     a     l     77    1800 or                                                                            1    0    "    "       1    0                                            more                                                 __________________________________________________________________________

EXAMPLE 2-2

Sb-doped SnO₂ (Powder d) was used as an electrically conductive powderand α-Al₂ O₃ (Powder i) was used as a white inorganic powder, the mixingratio being changed as shown in Table 5. Other procedures were carriedout in an analogous manner to Example 2-1 to obtain leader tapes.

The results are shown as Sample Nos. 40 to 48 in Table 5.

It is apparent from Table 5 that the use of only the white inorganicpowder results in unfavourable electrification property, but joint useof the white inorganic powder with the electrically conductive powderresults in improvement of the durability of the cleaning effect.

COMPARATIVE EXAMPLE 2-1

The leader tape, Sample No. 40 of Example 2-2 was subjected to anantistatic treatment by coating the surface thereof with an antistaticagent of quaternary ammonium salt type (commercial name: EFUKOORU 72manufactured by Matsumoto Yushi KK) in a proportion of 200 mg/m². Theproperties of the resulting leader tape are shown Sample No. 49 in Table5.

                                      TABLE 5                                     __________________________________________________________________________                                            After Running 10,000 Times                    Sb--doped                  Electri-               Electri-                    SnO.sub.2                                                                           α-Al.sub.2 O.sub.3                                                           Percent    Cleaning                                                                           fication          Cleaning                                                                           fication            Sample  Powder                                                                              Powder i                                                                           Phototrans-                                                                         Cramp                                                                              Effect                                                                             Property                                                                           Head Missing of                                                                            Effect                                                                             Property            No.     d (parts)                                                                           (parts)                                                                            mission (%)                                                                         Force (g)                                                                          (times)                                                                            (cm) Clogging                                                                           Coating Layer                                                                         (times)                                                                            (cm)                __________________________________________________________________________    Example 2-2                                                                   40       0    100  74    1800 or                                                                            1    10.0 no   no      1    10.0                                         more                                                 41      10    90   75    1800 or                                                                            1    2.0  "    "       1    2.0                                          more                                                 42      20    80   76    1800 or                                                                            1    0.5  "    "       1    0.5                                          more                                                 43      30    70   76    1800 or                                                                            1    0.5  "    "       1    0.5                                          more                                                 44      50    50   78    1800 or                                                                            1    0    "    "       1    0                                            more                                                 45      70    30   80    1800 or                                                                            1    0    "    "       1    0                                            more                                                 46      80    20   81    1800 or                                                                            1    0    "    "       1    0                                            more                                                 47      90    10   83    1800 or                                                                            2    0    "    "       2    0                                            more                                                 48      100    0   84    1800 or                                                                            2    0    "    "       3    0                                            more                                                 Comparison 2-1                                                                49       0    100  74    1800 or                                                                            1    1    yes          2    5.0                                          more                                                 __________________________________________________________________________

EXAMPLE 2-3

Using 50 parts of White Electrically Conductive Powder a and 50 parts ofWhite Inorganic Powder i, the procedure of Example 2-1 was repeatedexcept changing the total weight parts of Vinyl Chloride-Vinyl AcetateCopolymer, Polyester Polyurethane and Polyisocyanate and the weight partof the solvent without changing the relative proportion of the binder(70:20:10)

The results are shown as Sample Nos. 50 to 58 in Table 6.

It will clearly be understood from Table 6 that the amount of the binderis preferably about 15 to 600% by weight.

                                      TABLE 6                                     __________________________________________________________________________                                      After Running 10,000 Times                         Total                 Electri-               Electri-                         Amount of                                                                           Percent    Cleaning                                                                           fication          Cleaning                                                                           fication                  Sample Binder                                                                              Phototrans-                                                                         Cramp                                                                              Effect                                                                             Property                                                                           Head Missing of                                                                            Effect                                                                             Property                  No.    (parts)                                                                             mission (%)                                                                         Force (g)                                                                          (times)                                                                            (cm) Clogging                                                                           Coating Layer                                                                         (times)                                                                            (cm)                      __________________________________________________________________________    Example 2-3                                                                   50      10   70    1500 1    0    no   yes     1    0                         51      15   72    1550 1    0    "    somewhat                                                                              1    0                         52      25   74    1600 1    0    "    no      1    0                         53      50   76    1800 or                                                                            1    0    "    "       1    0                                            more                                                       54     100   78    1800 or                                                                            1    0    "    "       1    0                                            more                                                       55     200   79    1800 or                                                                            1    0    "    "       1    0                                            more                                                       56     400   80    1800 or                                                                            1    0    "    "       1    0                                            more                                                       57     600   80    1800 or                                                                            2    0.5  "    "       2    0.5                                          more                                                       58     800   81    1800 or                                                                            2    1.0  "    "       3    1.0                                          more                                                       __________________________________________________________________________

EXAMPLE 2-4

The leader tapes of Example 2-1, Example 2-2 and Compatative Example 2-1were slit in a width of 1/2 inch to obtain leader tapes for video tapes.The resulting leader tapes were combined with hubs in the order of 17 cmof Leader Tape, 247 m of Video Tape and 17 cm of Leader Tape. Theresulting VHS video cassette tapes were respectively subjected torunning 200 times in a VHS video deck for assessment of theperformances.

The results are shown as Sample Nos. 59 to 63 (Example 2-4) and asSample Nos. 64 and 65 (Comparison 2-2).

It is apparent from Table 7 that the leader tapes of the presentinvention are free from head clogging and increase of drop out as leadertapes for video tapes.

                                      TABLE 7                                     __________________________________________________________________________                  Amount of                                                               Electrically                                                                        Electrically                                                                          White                                                                              Amount of                                                                             Coating of                                 Sample  Conductive                                                                          Conductive                                                                            Inorganic                                                                          white Inorganic                                                                       Antistatic                                                                          Head Increase of                     No.     Powder                                                                              Powder (parts)                                                                        Powder                                                                             Powder (parts)                                                                        Agent Clogging                                                                           Drop Out                        __________________________________________________________________________    Example 2-4                                                                   59      a     50      i    50      no    no   no                              60      b     50      i    50      "     "    "                               61      c     50      i    50      "     "    "                               62      d     50      i    50      "     "    "                               63      e     50      i    50      "     "    "                               Comparison 2-2                                                                64      --    --      i    100     "     "    yes                             65      --    --      i    100     yes   yes  somewhat                        __________________________________________________________________________

Examples of the white resin powder used in the following Examples andComparative Examples are as follows:

m: Polyethylene Powder (mean grain size 2.5 μm; MP 110° C.; truespecific gravity 0.95)

h: Triazine type Resin Powder (mean grain size 2.5 μm).

EXAMPLE 3-1

    ______________________________________                                        Electrically Conductive Powder (as shown                                                               50      parts                                        in Table 8)                                                                   White Resin Powder (as shown in Table 8)                                                               50      parts                                        Vinyl Chloride-Vinyl Acetate Copolymer                                                                 70      parts                                        (composition: 87:13, degree of Polymerization)                                Polyester Polyurethane (molecular weight:                                                              20      parts                                        150,000)                                                                      Polyisocyanate (commercial name: Desmodur                                                              10      parts                                        L; solid content of 75% solution)                                             Methyl Ethyl Ketone      2700    parts                                        ______________________________________                                    

The above described components were well blended to prepare coatingcompositions for leader tapes and then processed in an analogous mannerto Example 1-1 to obtain leader tapes for compact cassettes.

The thus resulting leader tapes were subjected to assessment in ananalogous manner to Example 1-1, thus obtaining results as shown inTable 8 as Sample Nos. 66 to 75.

It is apparent from the results of Table 8 that the leader tapes usingthe electronconductive powders and white resin powders in combinationaccording to the present invention are more excellent in cleaning effectas well as antistatic effect and these effects are not changed evenafter running 10,000 times.

                                      TABLE 8                                     __________________________________________________________________________                                            After Runing 10,000 Times                    White                       Electri-               Electri-                   Electrically                                                                        White Percent    Cleaning                                                                           fication          Cleaning                                                                           fication            Sample Conductive                                                                          Resin Phototrans-                                                                         Cramp                                                                              Effect                                                                             Property                                                                           Head Missing of                                                                            Effect                                                                             Property            No.    Powder                                                                              Powder                                                                              mission (%)                                                                         Force (g)                                                                          (times)                                                                            (cm) Clogging                                                                           Coating Layer                                                                         (times)                                                                            (cm)                __________________________________________________________________________    Example 3-1                                                                   66     a     m     83    1600 or                                                                            1    0    no   no      2    0                                            more                                                 67     b     m     82    1600 or                                                                            1    0    "    "       2    0                                            more                                                 68     c     m     82    1600 or                                                                            1    0    "    "       2    0                                            more                                                 69     d     m     83    1600 or                                                                            1    0    "    "       2    0                                            more                                                 70     e     m     84    1600 or                                                                            1    0    "    "       2    0                                            more                                                 71     a     h     82    1800 or                                                                            1    0    "    "       2    0                                            more                                                 72     b     h     81    1800 or                                                                            1    0    "    "       2    0                                            more                                                 73     c     h     82    1800 or                                                                            1    0    "    "       2    0                                            more                                                 74     d     h     83    1800 or                                                                            1    0    "    "       2    0                                            more                                                 75     e     h     84    1800 or                                                                            1    0    "    "       2    0                                            more                                                 __________________________________________________________________________

EXAMPLE 3-2

Sb-doped SnO₂ (Powder d) was used as an electrically conductive powderand Triazine type Resin Powder (Powder h) was used as a white resinpowder, the mixing ratio being changed as shown in Table 9. Otherprocedures were carried out in an analogous manner to Example 3-1 toobtain leader tapes.

The results are shown as Sample Nos. 76 to 84 in Table 9.

It is apparent from Table 9 that the use of only the white resin powderresults in unfavourable electrification property.

COMPARATIVE EXAMPLE 3-1

The leader tape, Sample No. 77 of Example 3-2 was subjected to anantistatic treatment by coating the surface thereof with an antistaticagent of quaternary ammonium salt type (commercial name: EFUKOORU 72manufactured by Matsumoto Yushi KK) is a proportion of 200 mg/m².

                                      TABLE 9                                     __________________________________________________________________________                                           After Running 10,000 Times                     Sb--doped                                                                           Triazine                                                                            Percent       Electri-      Missing   Electri-                    SnO.sub.2                                                                           type Resin                                                                          Photo-                                                                             Cramp                                                                             Cleaning                                                                           fication                                                                           Head     of   Cleaning                                                                           fication            Sample  Powder d                                                                            Powder h                                                                            transmis-                                                                          Force                                                                             Effect                                                                             Property                                                                           Clog-                                                                             Head Coating                                                                            Effect                                                                             Property            No.     (parts)                                                                             (parts)                                                                             sion (%)                                                                           (g) (times)                                                                            (cm) ging                                                                              Scratch                                                                            Layer                                                                              (times)                                                                            (cm)                __________________________________________________________________________    Example 3-2                                                                   76       0    100   84   1800 or                                                                           1    10.0 no  no   no   2    10.0                                         more                                                 77      10    90    84   1800 or                                                                           1    2.0  "   "    "    2    2.0                                          more                                                 78      20    80    83   1800 or                                                                           1    0.5  "   "    "    2    0.5                                          more                                                 79      30    70    83   1800 or                                                                           1    0.5  "   "    "    2    0.5                                          more                                                 80      50    50    83   1800 or                                                                           1    0    "   "    "    2    0                                            more                                                 81      70    30    83   1800 or                                                                           1    0    "   "    "    2    0                                            more                                                 82      80    20    83   1800 or                                                                           1    0    "   "    "    2    0                                            more                                                 83      90    10    83   1800 or                                                                           2    0    "   "    "    3    0                                            more                                                 84      100    0    84   1800 or                                                                           2    0    "   some-                                                                              "    4    0                                            more              what                               Comparison 3-1                                                                85       0    100   84   1800 or                                                                           1    1.0      some-                                                                              "    1    5.0                                          more              what                               __________________________________________________________________________

EXAMPLE 3-3

Using 50 parts of Electrically Conductive Powder d and 50 parts of WhiteResin Powder h, the procedure of Example 3-1 was repeated exceptchanging the total weight parts of Vinyl Chloride-Vinyl AcetateCopolymer, Polyester Polyurethane and Polyisocyanate, as the binder, andthe weight part of the solvent without changing the relative proportionof this binder (70:20:10), thus obtaining leader tapes.

The results are shown as Sample Nos. 86 to 94, in Table 10.

It will clearly be understood from Table 10 that the amount of thebinder is preferably about 15 to 600% by weight.

                                      TABLE 10                                    __________________________________________________________________________                                    After Running 10,000 Times                                               Electri-               Electri-                        Total  Percent    Cleaning                                                                           fication          Cleaning                                                                           fication                    Sample                                                                            Amount of                                                                            Phototrans-                                                                         Cramp                                                                              Effect                                                                             Property                                                                           Head Missing of                                                                            Effect                                                                             Property                    No. Binder (parts)                                                                       mission (%)                                                                         Force (g)                                                                          (times)                                                                            (cm) Clogging                                                                           Coating Layer                                                                         (times)                                                                            (cm)                        __________________________________________________________________________    86   10    71    1500 1    0    no   yes     2    0.5                         87   15    76    1560 1    0    "    somewhat                                                                              2    0.5                         88   25    79    1600 1    0    "    no      2    0                           89   50    81    1800 or                                                                            1    0    "    "       2    0                                            more                                                         90  100    82    1800 or                                                                            1    0    "    "       2    0                                            more                                                         91  200    83    1800 or                                                                            1    0    "    "       2    0                                            more                                                         92  400    83    1800 or                                                                            1    0    "    "       2    0                                            more                                                         93  600    84    1800 or                                                                            2    0.5  "    "       3    0.5                                          more                                                         94  800    85    1800 or                                                                            2    1.0  "    "       4    1.0                                          more                                                         __________________________________________________________________________

EXAMPLE 3-4

The leader tapes of Example 3-1, Example 3-2 and Comparative Example 3-1were slit in a width of 1/2 inch to obtain cleaning leader tapes forvideo tapes. The resulting leader tapes were combined with hubs in theorder of 17 cm of Cleaning Leader Tape, 247 m of Video Tape and 17 cm ofCleaning Leader Tape. The resulting VHS video cassette tapes wererespectively subjected to running 200 times in a VHS video deck forassessment of the performances. The results are shown as Sample Nos. 95to 101 in Table 11.

It is apparent from Table 11 that the cleaning leader tapes of thepresent invention are free from head clogging and increase of drop outas leader tapes for video.

                                      TABLE 11                                    __________________________________________________________________________              Amount of                                                               Electrically                                                                        Electrically   Amount of                                                                            Coating of                                    Sample                                                                            Conductive                                                                          Conductive                                                                            White  White Resin                                                                          Antistatic                                                                          Head Increase of                        No. Powder                                                                              Powder (parts)                                                                        Resin Powder                                                                         (parts)                                                                              Agent Clogging                                                                           Drop Out                           __________________________________________________________________________    95  a     50      h      50     no    no   no                                 96  b     50      h      50     "     "    "                                  97  c     50      h      50     "     "    "                                  98  d     50      h      50     "     "    "                                  99  e     50      h      50     "     "    "                                  100 --    --      h      100    "     "    yes                                101 --    --      h      100    yes   yes  somewhat                           __________________________________________________________________________

EXAMPLE 4

The procedure of Example 2-3, Sample No. 54 was repeated but using 50parts of Sb-doped SnO₂ (Powder d) as an electrically conductive powder,25 parts of α-Al₂ O₃ (Powder i) as a white inorganic powder and 25 partsof Triazine type Resin Powder (Powder h) as a white resin powder, thusobtaining a leader tape having substantially the same properties excepta percent photo-transmission of 77, as those of Sample No. 54 of Example2-3.

When the surface state of a head used in the test was observed afterrunning 10,000 times by means of a microscope, however, some scratch wasfound in the running direction on the head surface in the case of SampleNo. 54, while no scratch was found in the case of this Example 4.

It will be understood from this result that the leader tape having acoating layer using jointly an electrically conductive powder, whiteinorganic powder and white resin powder scratches a head less than thetape containing no white resin powder.

As is evident from the foregoing illustrations and Examples, a magnetictape having the leader tape whose cleaning layer contains predominantlyan electrically conductive powder and binder according to the presentinvention is more excellent in cleaning effect as well as antistaticeffect, which effects are hardly deteriorated even after running 10,000times, and is more improved in effects of preventing head clogging andincrease of drop out, as compared with magnetic tapes of the prior art.

Moreover, a magnetic tape having the leader tape whose cleaning layercontains further a white inorganic powder in addition to the abovedescribed powders according to the present invention exhibits a moreimproved cleaning effect.

In a magnetic tape having the cleaning leader tape whose coating layercontains predominantly an electrically conductive powder, white resinpowder and binder according to the present invention, not only thecleaning effect can further be improved more than in the case of usingthe white electrically conductive powder, but there is given anadvantage that a head is hardly scratched because of the white resinpowder.

What is claimed is:
 1. A magnetic tape having a leader tape comprising acoating layer provided on one or both sides of a support at least oneend of said magnetic tape, characterized in that the coating layercontains, as predominant components, an electrically conductive powderhaving a mean grain size of 0.001 to 1 um and a volume resistance of 10⁰to 10⁸ Ω-cm and being at least one member selected from the groupconsisting of SnO₂, In₂ O₃, ZnO, BaTiO₃, and SrTiO₃ which have beenmodified to provide electrical conductivity and a binder and the leadertape has a percent photo-transmission of at least 20%.
 2. The magnetictape having a leader tape as claimed in claim 1, wherein theelectrically conductive powder is at least one member selected from thegroup consisting of SnO₂, In₂ O₃, BaTiO₃, SrTiO₃ and (Ba, Sr) TiO₃ whichare doped with an element of different valence to increase theconductivity.
 3. The magnetic tape having a leader tape as claimed inclaim 1, wherein the electrically conductive powder powder is at leastone member selected from the group consisting of ZnO, BaTiO₃ and SrTiO₃into which oxygen defects are introduced.
 4. The magnetic tape having aleader tape as claimed in claim 1, wherein the electrically conductivepowder is at least one member of white pigments and extenders which arecoated with at least one member of electrically conductive materials. 5.The magnetic tape having a leader tape as claimed in claim 4, whereinthe electrically conductive materials are SnO₂, In₂ O₃, ZnO, BaTiO₃ andSrTiO₃ which have electrical conductivity, SnO₂, In₂ O₃, BaTiO₃, SrTiO₃and (Ba, Sr)TiO₃ which are doped with an element of different valence,and ZnO, BaTiO₃ and SrTiO₃ into which oxygen defects are introduced. 6.The magnetic tape having a leader tape as claimed in claim 1, whereinthe binder is in a proportion of 15 l to 600 parts by weight to 100parts by weight of the powders.
 7. The magnetic tape having a leadertape as claimed in claim 1, wherein the support is a non-magneticflexible support having a haze of at most 80%.
 8. The magnetic tapehaving a leader tape as claimed in claim 1, wherein the binder is atleast one member selected from the group consisting of vinylchloride-vinyl acetate copolymers, vinylidene chloride copolymers,nitrocellulose, polyester resins, polyurethane resins andpolyisocyanates.
 9. The magnetic tape having a leader tape as claimed inclaim 1, wherein the coating layer has a thickness of 0.05 to 20 um. 10.A magnetic tape having a leader tape comprising a coating layer providedon one or both sides of a support, at at least one end of the tape,characterized in that the coating layer contains, as predominantcomponents, an electrically conductive powder having a mean grain sizeof 0.001 to 1 um and a volume resistance of 10⁰ to 10⁸ Ω-cm and being atleast one member selected from the group consisting of SnO₂, In₂ O₃,ZnO, BaTiO₃, and SrTiO₃ which have electrically conductivity, a whiteinorganic powder having a particle size range of 0.01 to 5 um and abinder and the leader tape has a percent photo-transmission of at least20%.
 11. The magnetic tape having a leader tape as claimed in claim 10,wherein the white inorganic powder has a grain size of 0.01 to 3 μm. 12.The magnetic tape having a leader tape as claimed in claim 10, whereinthe white inorganic powder is at least one member selected from thegroup consisting of alumina, titanium oxide, zinc sulfide, lithopone,barium sulfate, zinc oxide, aluminum silicate and silicon oxide.
 13. Themagnetic tape having a leader tape as claimed in claim 10, wherein thewhite inorganic powder is used in a proportion of up to 950 parts byweight to 100 parts by weight of the electrically conductive powder. 14.A magnetic tape having a leader tape comprising a coating layer providedon one or both sides of a support, at least one end of the tape,characterized in that the coating layer contains, as predominantcomponents, an electrically conductive powder having a mean grain sizeof 0.001 to 1 um and a volume resistance of 10⁰ to 10⁸ Ω-cm and being atleast one member selected from the group consisting of SnO₂, In₂ O₃,ZnO, BaTiO₃, and SrTiO₃ which have electrically conductivity, a whiteresin powder having a particle size in the range of 0.1 to 10 um and abinder and the leader tape has a percent photo-transmission of at least20%.
 15. The magnetic tape having a leader tape as claimed in claim 14,wherein the white resin powder has a grain size of 0.1 to 3 μm.
 16. Themagnetic tape having a leader tape as claimed in claim 14, wherein thewhite resin powder is at least one member selected from the groupconsisting of acetylcellulose, polyethylene, polypropylene, polyesterresin, polyamide resin, polyacrylonitrile resin, polyacetyl resin,phenol resin, urea resin, triazine resin, malamine resin and epoxy resinpowders.
 17. The magnetic tape having a leader tape as claimed in claim14, wherein the white resin powder is used in a proportion of up to 450parts by weight to 100 parts by weight of the electrically conductivepowder.
 18. A magnetic tape having a leader tape comprising a coatinglayer provided on one or both sides of a support at at least one end ofsaid magnetic tape, characterized in that the coating layer contains, aspredominant components, at least one electrically modified conductivepowder selected from the group consisting of SnO₂, InO₃, ZnO, BatiO₃ andSrTiO₃, a white inorganic powder, a white resin powder and a binder andthe leader tape has a percent photo-transmission of at least 20%.